1. Field of the Invention
The present invention relates to an improved system and method for locating optimal routes between source and destination nodes in a communications network, in particular, a wireless ad-hoc communications network. More particularly, the present invention relates to a system and method for identifying optimal routes between source and destination nodes in a communications network by estimating the a posteriori packet completion rate.
2. Description of the Related Art
Wireless communication networks, such as mobile wireless telephone networks, have become increasingly prevalent over the past decade. These wireless communications networks are commonly referred to as “cellular networks”, because the network infrastructure is arranged to divide the service area into a plurality of regions called “cells”. A terrestrial cellular network includes a plurality of interconnected base stations, or base nodes, that are distributed geographically at designated locations throughout the service area. Each base node includes one or more transceivers that are capable of transmitting and receiving electromagnetic signals, such as radio frequency (RF) communications signals, to and from mobile user nodes, such as wireless telephones, located within the coverage area. The communications signals include, for example, voice data that has been modulated according to a desired modulation technique and transmitted as data packets. As can be appreciated by one skilled in the art, network nodes transmit and receive data packet communications in a multiplexed format, such as time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, or frequency-division multiple access (FDMA) format, which enables a single transceiver at a first node to communicate simultaneously with several other nodes in its coverage area.
In recent years, a type of mobile communications network known as an “ad-hoc” network has been developed. In this type of network, each mobile node is capable of operating as a base station or router for the other mobile nodes, thus eliminating the need for a fixed infrastructure of base stations. Details of an ad-hoc network are set forth in U.S. Pat. No. 5,943,322 to Mayor, the entire content of which is incorporated herein by reference.
More sophisticated ad-hoc networks are also being developed which, in addition to enabling mobile nodes to communicate with each other as in a conventional ad-hoc network, further enable the mobile nodes to access a fixed network and thus communicate with other mobile nodes, such as those on the public switched telephone network (PSTN), and on other networks such as the Internet. Details of these advanced types of ad-hoc networks are described in U.S. Pat. No. 7,072,650 entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networks”, granted on Jul. 4, 2006, in U.S. patent application Ser. No. 09/815,157 entitled “Time Division Protocol for an Ad-Hoc, Peer-to-Peer Radio Network Having Coordinating Channel Access to Shared Parallel Data Channels with Separate Reservation Channel”, filed on Mar. 22, 2001, now U.S. Pat. No. 6,807,165, and in U.S. patent application Ser. No. 09/815,164 entitled “Prioritized-Routing for an Ad-Hoc, Peer-to-Peer, Mobile Radio Access System”, filed on Mar. 22, 2001, now U.S. Pat. No. 6,873,839, the entire content of each being incorporated herein by reference.
As can be appreciated by one skilled in the art, since certain nodes of the ad-hoc network are mobile, it is necessary for the network to maintain connectivity with those nodes. Transmitted data packets typically “hop” from mobile device to mobile device, creating a transmission path, or route, until reaching a final destination. However, transmission paths between mobile devices are often subject to change as devices move, therefore ad-hoc network communication must be able to adapt to achieve optimum performance while addressing the limited capabilities and capacities of mobile individual devices.
In a typical wireless communication network, the number of hops between the source and the destination is used as the routing metric, with the route having a lesser number of hops typically being a more preferred route. However, this can lead to selection of un-optimal routes, as there can be a better route with a greater number of hops but better link quality or data rate.
Examples of types of routing protocols are described in U.S. Pat. No. 5,412,654, and in U.S. Provisional Patent Application Ser. No. 60/476,237 referenced above, the entire contents of both documents being incorporated herein by reference. In these techniques, each node calculates a route metric to its destination, possibly using alternate routes. The aim is to select the best route by selecting the route with the lowest metric.
As described in U.S. Pat. No. 5,412,654, the metric is simply the number of hops. However, as described in U.S. Provisional Patent Application Ser. No. 60/476,237, the metric is a more elaborate value and relies on a “link reliability” calculation that is based on signal strength. The “link reliability” is a more refined component of a routing metric because, instead of adding “1” for each hop, the algorithm adds an integer value which is larger (e.g., 20) if the link cannot be used to its fullest potential. Thus, each node assigns an integer value which is minimal (e.g., 1) for the best radio links and larger (e.g., 20) if the radio link is degraded due to, for example, multipath, fading, congestion, distance, shadowing, interference, and so on. Other examples of techniques for determining link quality are set forth in published U.S. Patent Application No. 2004/0022223 and in U.S. Pat. No. 6,522,881, the entire contents of both being incorporated herein by reference.
Although the technique described in the provisional patent application referenced above is suitable for many radio networks, including wireless ad-hoc peer-to-peer networks, certain drawbacks of the technique may become apparent when used in a network comprising mainly low-cost wideband radios. For example, sporadic traffic errors such as those encountered in a normal radio channel (such as static multipath) do not usually cause the link reliability to be adjusted, since signal strength and signal-to-noise ratio are unaffected. Also, in the particular case of a fading channel, the low-cost wideband radio is not able to properly track the variations in signal strength that are encountered at the receiver's end. This is partly due to the fact that the measurement depends on the sensitivity of the radio, and partly due to the fact that the receiver can only make sporadic measurements (such as one particular point in time when the packet is received), which provides partial information about actual signal strength variations.
Accordingly, a need exists for an improved system and method for discovering optimal routes between source and destination nodes in a communications network in an efficient way using factors other than the number of hops or received signal strength as the sole metrics.